The present invention relates to a receiving section of a telephone with suppression of interference that is activated and deactiveated by switching on/off or the receiving section.
The signal received by a telephone of a digital telephone network is first demodulated and then processed in digital form, is converted into analog form and, finally, is amplified and applied to an electroacoustic transducer.
The receiving section of a telephone can be represented schematically as shown in FIG. 1. A demodulated digital signal RX-IN produced by a receiving and demodulation unit 10 is applied to the input of a circuit unit 11 in which it is converted into analog form and filtered and its voltage is amplified. The analog signal output from the unit 11 is applied to the input of a power amplifier 12, which can transfer the signal to an electroacoustic transducer 13 such as an electromagnetic unit or loudspeaker with sufficient electrical power for the operation thereof. The gain of the power amplifier 12 is determined by the ratio between the resistance of a feedback resistor R2 connected between the inverting input and the output of the amplifier and the resistance of a resistor R1 connected between the output of the unit 11 and the inverting input of the amplifier.
To prevent power dissipation when the telephone is not in use, circuit means which respond to an external activation or deactivation control are provided both in the unit 11 and in the amplifier 12. This function is represented by the connections of the unit 11 and of the amplifier 12 to a terminal to which a single-bit digital signal PD (power down) can be applied. By convention, the signal PD=1 corresponds to a zero current-absorption state and the signal PD=0 permits absorption of current from the supply (power up).
As is known, each time there is a change from the xe2x80x9cpower downxe2x80x9d state to the xe2x80x9cpower upxe2x80x9d state or vice versa, both the signal-processing unit and the power amplifier are subject to an abrupt voltage and current transition before reaching the steady operating state. During this transition, electrical disturbances with frequency components in the acoustic band are produced and are translated into audible and annoying noises in the electroacoustic transducer.
To prevent these problems, various measures that provide for the connection of active and passive components in series and/or in parallel with the transducer have been adopted to filter out the undesired electrical disturbances. However, a solution of this type requires fairly bulky components outside the power amplifier, which is normally in the form of an integrated circuit.
It has also been proposed to use processing units and amplification units with balanced inputs and outputs in order to cancel out at least some of the possible disturbances, particularly so-called common-mode disturbances. However, according to the prior art, the use of balanced amplification units was considered advantageous only when the electroacoustic transducer could be connected between the two balanced outputs. When this was not possible because one of the terminals of the transducer had to be connected to a fixed power terminal, for example to the ground of the circuit, it was not possible to make use of the intrinsic advantage of the balanced amplifier.
In accordance with one disclosed embodiment of the present invention a receiving section for a telephone is provided that has a final stage and an electroacoustic transducer having one of its terminals connected to the ground terminal, and which is free of the above-described disturbances and does not require filtering components.
In accordance with another disclosed embodiment of the present invention, a receiving section of a telephone is provided, including a final stage; an electroacoustic transducer having a first terminal connected to a ground terminal of the receiving section; a unit for controlling switching on/off; a voltage source having a reference voltage terminal; a switch having a first position and a second position in order to connect a second terminal of the transducer selectively via a capacitor to the reference-voltage terminal or to an output terminal of the final stage, respectively; and a controller that responds to signals from the unit for controlling switching on/off in order to activate or deactivate the final stage, the reference voltage source, and the switch in accordance with a predetermined time program.